1. Field of the Invention
The present invention relates to a clutch master cylinder for use in a clutch release mechanism, and more particularly to a clutch master cylinder of the variable valve lift type which includes a valve seat arranged between inlet and outlet ports, and a valve body associated with a piston to cooperate with the valve seat in response to axial movement of the piston so as to interrupt the communication between the inlet and outlet ports.
2. Description of the Prior Art
As is illustrated in FIG. 6, a conventional clutch master cylinder of this kind comprises a cylinder body 2 provided thereon with a fluid reservoir tank T and therein with an inlet port 2a.sub.1 in communication with the reservoir tank T and an outlet port 2a.sub.2 for connection to a slave cylinder in a clutch release mechanism, a closure plug 1 adjustably threaded into one end of the cylinder body 2 and fastened in place by a lock nut 3, the closure plug 1 being formed at the inner end thereof with an annular valve seat 1a arranged between the inlet and outlet ports 2a.sub.1 and 2a.sub.2, a piston 4 axially slidably disposed within the cylinder body 2 and arranged to be moved by a push-rod in depression of a clutch pedal, and a valve body 5 associated with the piston 4 to cooperate with the valve seat 1a in response to axial movement of the piston 4 so as to interrupt the communication between the inlet and outlet ports 2a.sub.1 and 2a.sub.2.
In such a construction of the conventional clutch master cylinder as described above, it is advantageous that the axial space between the valve seat 1a and the valve body 5 (hereinafter referred to a lift amount of the valve body 5) can be adjusted by back and forth adjustment of the closure plug 1. This enables adjustable operation of the clutch release mechanism in accordance with defacement of the clutch disc so as to ensure the operator's feel in depression of the clutch pedal. It has been, however, experienced that when the lock nut 3 is loosened to permit outward movement of the closure plug 1 for adjustment of the lift amount of the valve body 5, the closure plug 1 is excessively retracted from the cylinder body 2 because of no provision of any stopper for restricting outward movement of the closure plug. As a result, the lift amount of the valve body 5 is in excess an allowable amount, and annular seal members 6 and 7 for the closure plug 1 are damaged by abutment against a threaded portion 2c and an annular groove 2b formed in the cylinder body 2. If the closure plug 1 is excessively threaded into the cylinder body 2, the lift amount of the valve body 5 will be determined less than the allowable amount, and the threaded portion 1b of closure plug 1 will be engaged with an unthreaded portion 2d in the cylinder body 2 so as to cause stick and wear of the closure plug 1 at its intermediate portion. If the valve seat 1a is abutted against the valve body 5 in adjustment of the closure plug 1, a fluid under pressure will be shut up in the pressure chamber of cylinder body 2.